1. Field of the Invention
The present invention relates to methods for preparing esters of cyclohexane polycarboxylic acids, and more particularly, to a method for preparing esters of cyclohexane polycarboxylic acids by hydrogenating esters of benzene polycarboxylic acids.
2. Description of Prior Art
In the field of plasticization for PVC, esters of benzene polycarboxylic acids are widely used as the plasticizer that endows PVC products with improved softness and toughness. However, such esters of benzene polycarboxylic acids as dibutyl phthalate, diisooctyl phthalate (DIOP) and diisononyl phthalate (DINP) are environmental hormone harmful to human health and have been legally limited to or prohibited from use in many countries, while esters of cyclohexane polycarboxylic acids are instead used as the plasticizer for PVC.
Conventionally, esters of cyclohexane polycarboxylic acids have been made through either esterification or hydrogenation. For preparing esters of cyclohexane polycarboxylic acids through esterification, a cyclohexane polycarboxylic acid or a cyclohexane polycarboxylic anhydride is esterified with an aliphatic monohydric alcohol. However, since cyclohexane polycarboxylic acids and cyclohexane polycarboxylic anhydrides are expensive, this process is unfavorable to industrial production.
On the other hand, preparing esters of cyclohexane polycarboxylic acids through hydrogenation, as taught by U.S. Pat. No. 2,070,770, involves hydrogenating an ester of a benzene polycarboxylic acid in the presence of a hydrogenation catalyst under a raised pressure.
As shown in FIG. 1, in a known hydrogenation process for making esters of cyclohexane polycarboxylic acids, an apparatus implemented has a hydrogenation tank 10, provided therein with a traditional impeller mixing device 20. The impeller mixing device 20 uses a rotatory shaft 21 to drive vanes 22 at the terminal of the rotatory shaft 21 to rotate and thereby stir a solution of an ester of a benzene polycarboxylic acid (hereinafter referred to as the reaction liquid) 30. While the vanes 22 rotate, a hydrogen nozzle 60 immersed in the reaction liquid 30 introduce a high-pressure hydrogen gas into the reaction liquid 30, so that the rotating vanes 22 promote the contact between hydrogen gas and the reaction liquid 30, making the reaction liquid 30 get hydrogenated in the presence of the hydrogen gas and a catalyst, thereby obtaining the desired ester of the cyclohexane polycarboxylic acid.
Such a hydrogenation tank 10, however, is less effective in increasing the contact between hydrogen gas and the reaction liquid 30, so the yield of the ester of the cyclohexane polycarboxylic acid as a production of hydrogenation is relatively low. For improving the productivity, the hydrogenation tank 10 has to be made for high-pressure operation and thus unavoidably requires higher costs in fabrication, operation and maintenance, being unfavorable to industrial production as well.